The general guidelines in respect of laser safety (for example in accordance with EN 60825, ANSI 2136 or corresponding to the “International Commission on Non-Ionizing Radiation Protection”) demand that a laser light source can only be operated such that it does not pose any hazards. In this case, depending on the wavelength emitted, limit values for the thermal power density or the energy density need to be adhered to. Eye safety is of primary importance in laser devices since the eye, owing to its function, reacts most sensitively to electromagnetic radiation and damage to the retina or cornea can occur as a result of direct, indirect or scattered laser beams. This applies in particular since studies have shown that, even in the case of visible light, it generally cannot be assumed that the lid closure reflex protects the eye. Laser devices need to be classified corresponding to these specifications and to be identified correspondingly, which could also include safety testing and certification.
In the case of a large number of laser devices, for example electro-optical measurement devices such as laser distance measuring devices or laser levelers, laser projectors, laser scanners, etc., the emergence of the laser light from the housing of the device is absolutely necessary for functional reasons. A minimum optical energy is also often required for functional reasons, and this minimum optical energy would be above the nonhazardous limit if this were to be emitted in the continuous-wave operating mode. Therefore, the lasers in such devices are usually operated in pulsed fashion. In order to guarantee eye safety, the laser sources need to be equipped with corresponding protective measures in order to adhere to relevant standards and specifications. This applies not only in the conventional operating case, but also under so-called “single-fault conditions”, which all cover any fault scenarios in which a single fault occurs on its own (for example failure of individual component parts, short circuit, conductor track breakage, etc.). In this case, even in the event of failure of any component, for example, it is necessary to ensure that, despite the occurrence of such a single fault, the laser power occurring falls to below the upper power limit of the corresponding laser class within fractions of a second in order to rule out damage to the eye. During pulsed operation, care should be taken, using special measures, to ensure that continuous emission with the peak pulse values which are above the limit values can also be ruled out in the event of a fault. Such single-fault tests need to be correspondingly carried out and verified by the manufacturer of the laser device.
Depending on the laser class and the application case, two-fault or multiple-fault failsafety can also continue to be required, in which, even in the event of the simultaneous occurrence of more than one fault, evidence needs to be given of the safety to the effect that the emitted laser power does not in any way exceed the limit values and therefore a risk to the user can be ruled out. The single-fault failsafety described in this document also represents a basic precondition which in any case needs to be met in these cases, which basic precondition can be developed by corresponding further measures.
The conventional solution for producing laser safety is the direct monitoring of the emitted laser power with the aid of a monitor diode. In this case, some of the emitted laser light is directed onto a photosensitive element, for example a photodiode or a phototransistor, which provides an electrical signal which is dependent on the light intensity. A monitoring circuit, for example in the form of a microcontroller or a discrete circuit (possibly even a mandatory one in the case of safety-relevant circuit parts), can thus monitor the presently emitted laser power and possibly disconnect the laser in the event of a limit value for the emitted laser energy or laser power being exceeded.
For example, documents EP 0 314 390, U.S. Pat. No. 5,287,375, EP 0 780 937, EP 0 664 591 and EP 0 597 644 disclose a wide variety of laser driver circuits which all have a monitoring circuit with a monitor diode, to which some of the light emitted by the laser diode is applied and with the aid of which the present output power of the laser diode can be determined. In the event of a fault, for example in the event of a short circuit of a power transistor in the output stage of the laser diode actuation, the supply of electrical energy to the laser diode can be reduced or suppressed on the basis of this information by means of a correspondingly designed safety circuit.
Owing to the general nature of the laser safety provisions or else owing to regulation of the output power of the laser diode which is often required depending on the application, a large number of commercially available laser diode components are already equipped with a corresponding monitor diode. Laser diodes also often have considerable manufacturing tolerances, temperature dependencies or ageing effects, which can be compensated for by determination of the actual optical output power. The circuitry complexity for such monitoring is really high and also correspondingly cost-intensive. The complexity of evaluating the analog and often interference-susceptible signal of the photosensitive monitoring element, often in the form of a photodiode, thus remains. Often such a safety circuit also needs to be implemented in a manner which is safe and certifiable, supported by corresponding evidence, which can additionally increase the complexity in terms of circuitry, in particular since the realization of safety-relevant circuit parts is often linked to stringent conditions and documentation specifications.
In addition, during pulsed operation of laser diodes, during which only very short light pulses with a high peak power are emitted, owing to the steep switching edges occurring in the process in the electronic circuit and high peak currents, corresponding electromagnetic interference signals (crosstalk, etc.) are to be expected, which disrupt the evaluation (which usually takes place at high resistances) of the measurement signals of a monitor diode and can complicate this. The short pulse duration of the emitted light and the corresponding monitor signals can also make monitoring of the actually emitted average light energy more difficult.
In this case, the laser pulses are emitted multiply in packets of pulses, so-called “bursts”, as is described, for example, in EP 01 957 668. After a packet with a number of short laser pulses in quick succession, there is a dead time in which no laser emission takes place and which is markedly longer than the intervals between the pulses within the packet.
JP 7 079042 discloses the use of a pulse-shaping network in order to supply a current pulse which does not have any interference to a laser diode.
JP 2008 227408 describes an energy-efficient increase in a DC voltage by means of a step-up converter for supplying power to a series circuit comprising a plurality of light-emitting diodes with a forward voltage which is greater than the DC voltage.